Detectives have identified a suspect in the blast which destroyed a passenger bus in the Russian city of Togliatti, killing eight people and injuring 50, Russia's chief investigator said.

"A definite suspect has been identified, his apartment was searched," Alexander Bastrykin, the federal head of the investigations committee, told Vesti-24 television.

"We have every reason to believe that this person, willingly or not, performed actions which led to the explosion in the bus."

Mr Bastrykin also said that detectives had found aluminium wire and nails similar to those used in the bus bomb at the home of the suspect, whom he said was travelling on the vehicle when the blast happened.

The presidential envoy to the Volga region, Alexander Konovalov, said the blast detonated at a height of about 1.2 metres above the floor of the bus, suggesting it could have been the work of a suicide bomber.

But Mr Bastrykin said this had not been confirmed.

"There can be two explanations: this was either a terrorist act motivated by some reason, not necessarily by nationalism, or a careless handling of explosive substances which could blow up during motion," he told Russian television.

Authorities declared a day of mourning in Togliatti, a car-making city 1,000 kilometres south-east of Moscow best known as the home of the Lada car. It is named after Italian Communist leader Palmiro Togliatti.

The blast came just over a month before Russia votes in parliamentary elections widely seen as a referendum on President Vladimir Putin's nearly eight years in power.

Russian media expressed concern that the Togliatti blast, which follows a bomb attack in August on a Moscow-St. Petersburg express train, might signal the beginning of a fresh wave of attacks.

"The Izvestia daily asked whether it was "the beginning of a new terror war".